I have played zombie games using AR:SE, Zombie Apocalypse, Zombie Plague, Dead of Night, Dead Walk Again, All Things Zombie (both versions),and a couple of other systems. While I like the simplicity of the AR:SE system and some of the others I am routinely drawn back to ATZ, yes it takes at the very least a couple games to grasp the mechanics, one of the nice features of it is that in ATZ if it is something that someone can do in real life, you can pretty much do it in the game, all you need to do is pass a test to do it, and ATZ also has great support, if you are stuck on understanding how a rule works, post over on the Two Hour Wargames yahoo group and you will usually have your answer from either Ed (the creator of ATZ) or one of the many other knowledgeable players. Ed also fully supports players, for their own personal uses, in creating their own house rules, modifying the rules, excluding rules, etc... to help make your games work for you.
For ATZ you have Better Dead then Zed core rules, Haven expansion/supplement introducing new rules and scenarios, and now I, Zombie introducing more new rules in addition to new zombie types and a new scenario setting. Ed is hoping to release another book here soon called After the Horsemen which is suppose to further flesh out ATZ in playing well after the outbreak and the society has collapsed into a Post Apocalyptic world. I also believe that After the Horsemen will be including some if not most the core material from the short lived All Things Horror rules Ed had released, such as the rules for Vampires and other monsters.